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In our FCC 4 PDT A,B,C,D are provide to show RCSV Differential pressure , all PDTs are N2 purged with flow orifice and flow orifice bypass. from some days we are facing dp problems in C and D. If we are open flow orifice bypass of C and D than DP maintain and when we are close flow orifice bypass DP disturb of C and D.
What may be reason?
 
Answers
15/01/2018 A: Eric Hennings, Technip Stone & Webster, EHennings@technip.com
RCSV delta P’s are monitored by difference between pressure of upstream and downstream purge gas. RO’s are provided on each tap to achieve small but constant flow of purge gas. If purge rate is too low, then catalyst may backflow and choke the purge. If purge rate is too high, then there will be increased pressure drop in the purge line, giving a false signal to the DPT. Note that excessive purge can result in either HH or LL DPT reading, depending on whether the excess purge is on the upstream or downstream tap.
The RO is generally quite small opening, and so a strainer is provided in the purge gas supply to prevent choking. Opening of RO bypass is only used to clean the purge gas, and then returned to normal. Note DCS instruments must be bypassed during this remediation. If the RO bypass valve is not completely closed, excess frictional pressure can occur, resulting in “false” signal. As a check on purge gas rate, the last valve into process can be blocked and reaming valve opened to atmosphere.